As a dilution sampling system that dilutes collected exhaust gas with diluent gas such as air to measure the concentration of a component contained in the exhaust gas, a constant volume dilution sampling apparatus (CVS) is widely used.
In the CVS, a critical flow rate of a critical flow venturi constituting a constant flow rate mechanism is set such that a ratio at which the exhaust gas is diluted with the diluent gas is increased to the extent of preventing moisture in the exhaust gas from condensing, for example, 10 to 20 times. On the other hand, in the case of increasing the dilution ratio, the concentration of the component contained in the diluted exhaust gas is decreased, and therefore measurement accuracy is deteriorated due to a measurement error.
For this reason, in the past, the critical flow rate of the critical flow venturi has been set so as to make the dilution ratio as small as possible to the extent of preventing the moisture in the exhaust gas from condensing. Specifically, on the basis of displacement, maximum vehicle speed (engine rotation speed), and the like of an engine (vehicle) as a test object, the critical flow rate of the critical flow venturi is set from user's intuition and empirical rule.
However, depending on the type of an engine (vehicle), a test condition, or the like, an amount of moisture contained in exhaust gas is different, and therefore as described above, in the case of setting the critical flow rate of the critical flow venturi on the basis of the user's intuition and empirical rule, there occurs a problem that the exhaust gas cannot be diluted to the extent of preventing the moisture in the exhaust gas from condensing, or the dilution ratio of the exhaust gas is excessively increased.
Also, in the case of using air as the diluent gas, an amount of moisture contained in the air causes an error, and as described above, in the case of setting the critical flow rate of the critical flow venturi on the basis of the user's intuition and empirical rule, there occurs a problem that the exhaust gas cannot be diluted to the extent of preventing the moisture in the exhaust gas from condensing, or the dilution ratio of the exhaust gas is excessively increased.